Architects and designers love to interrupt straightness, corners and edges with soft curves.

This is why, in addition to round interior glass products and accessories, curved glass façades also exist.

Gravity bending

In building envelope applications, glass is generally bent through a thermal gravity process.
By the middle of the 19th century, architects were bending glass, a technique developed in England, and this exists today in a slightly modified form.

The procedure is as follows: a glass pane is laid over a bending form and heated to 550 - 620 °C in the bending oven. Having reached the softening temperature, the plain pane descends (through gravity) slowly into the bending mould and adopts its shape. The subsequent cooling down phase defines the shape of the glass. Slow cooling, free from residual stress, produces a glass which can be further processed, whereas fast cooling creates a partially or fully tempered glass, which is not suitable for further processing.

Advantages of gravity bending.

Good optical quality (no roller wave distortions).

No anisotropies.

All shapes (cylindrical and 2-axis) are possible.

Concave and convex shapes with coatings are possible.

A challenge is typically the long production time (heating up the box furnace + holding time + annealing time). This must be taken into account when discussing production capacities and lead times.

Gravity curved glass is not safety glass (no tempering or heat strengthening process). If safety requirements are to be considered, the curved glass panes can be laminated after the bending process. A pre-condition here is that all lites to be laminated are placed at the same time onto the mold in order to ensure the identical shape. This avoids stress in the laminate and delamination.

Bending toughened glass

During the process, the glass becomes curved and either fully tempered or heat strengthened at the same time. The treatment takes place in similar production facilities as those used for flat tempered or heat strengthened glass.
After the heating process in the furnace, the soft glass is mechanically curved (pushed) in the flexible quench zone while strengthening and cooling.

The cycle times for bending the glass are comparable to flat tempered or heat strengthened glass. The main problems can often be anisotropies and optical distortions caused by the production process. Another characteristic of this process is the limitation to cylindrical shapes and concave shapes when coatings are involved.

Bending shapes

A distinction is generally made between bent glass, slightly bent glazing with a bending radius of more than two metres and severely bent glass with small radii. Moreover there is a difference between glass which is bent cylindrically and spherically. Cylindrically bent glass is bent along one axis, and spherically over two axes.

Float glass is, in principle, suitable for all these bending shapes. Due to the production technology involved, fully tempered and heat strengthened glass are used primarily for cylindrical bending.

The smallest possible bending radius is approx. 100 mm for glass with a thickness < 10 mm and about 300 mm for glass > 10 mm thick. These possibilities depend on the experience and technical capabilities of the glass bending company and should be discussed in advance.

Requirements

Generally, bent glass is not a regulated building material, and it should ensure functionality such as thermal insulation, solar and noise protection. In addition, it must meet the requirements of building laws, such as fall prevention measures and load bearing regulations, to the same extent as plane glass. To verify this and be allowed to install bent glasses, manufacturers must provide approvals by local building authorities case by case or an ETA (European Technical Approval) in Europe before starting the construction. Comprehensive European standards for bent glass are currently not available, but full usability should be documented in all cases. The international standard ISO 11485 Glass in building – Curved glass describes terminology and definitions in part 1 and quality in part 2.

Suitable glass types

In principle, all glass types that are suitable for heat treatment are bendable. However, some restrictions apply to glass equipped with functional coatings. Typically, intensive feasibility studies must be carried out to determine the limitations related to the particular coating types, bending shapes and techniques.

Most of Guardian’s architectural sputter coatings are suitable for gravity and toughened bending or have a bendable version. This applies for all SunGuard solar control types, heat treatable ClimaGuard thermal insulating glass and Clarity anti-reflecting glass.

Today, in most cases desired glass types for a building are applicable in planar and curved parts of a façade too. This fulfills the architect‘s wish for ensuring a homogenious appearance of the whole building.

Exact descriptions of the dimensions are required for the shape determination of bent glass. In addition to thickness of the glazing, the height of the panes and the width of at least another two of the five dimensions needs to be determined in the following drawing for inner and outer execution. It should always be noted that, with the exception of the opening angle, all data refers to the same surface (concave = inside, convex = outside).

The standard bend is the cylindrical execution referred to in the definitions. All other geometrical shapes, such as spherical bends, should be estimated by an exact drawing, so that shape and size can be well determined. Linear elongations of cylindrical shapes (b1, b2) are to be displayed separately.

Specifics

Special tolerances and production shaping conditions, which should be strictly considered, apply to bent glasses:

Local optical distortions

The local distortions of fully tempered and heat strengthened glass may differ from the specifications for plain glasses, as glass geometry, size and thickness may have a greater influence on bending than with the plain design. These should be agreed in advance with the potential glass bending company in all cases.

Outline precision

Outline precision means the accuracy of bending. This should be within a tolerance range of ± 3 mm in relation to the target contour so that the glass can be processed further without any difficulties (Guideline on thermally curved glass for building applications – BF-Bulletin 009).

Torsion (twist deviation)

Torsion describes the exactness to the plane parallelism of the edges or unbent edges. In this case, the largest irregularity after bending should also not exceed ± 3 mm per metre of glass edge (Guideline on thermally curved glass for building applications – BF-Bulletin 009).

Edge displacement

Diverging from the specifications of plain laminated and insulating glass, the displacement at the edges may increase after bending. It is absolutely necessary to find common conformity in advance.

Tangential junctions

The tangent is the straight line which has its origin in a particular point of the curve. Thereby the line is perpendicular compared with the bent radius of the curve. Without this tangential transition there would be a sharp angle at this spot, which can be achieved with glass, but is not advisable. There are normally larger tolerances at the sharp angle than with tangential transitions.

Static specifics

Static specifics

The deformation and mechanical stress of a bend can be defined through finite element models with the aid of the shell theory. The curvature, depending on installation conditions in the case of monolithic glass, can have a positive effect due to the shell bearing impact, namely in the direction of thinner glasses. Insulating glass, however, does not achieve this effect as readily. The curvature of the glass means that the bending strength is increased and, consequently, extremely high climatic loads can arise. This must be considered, particularly when units have tangential attachment pieces at a curvature. This can result in broader edge seals that affect later glass installation.

Cold-bending

This technique allows designers to create continuously smooth glass surfaces (as opposed to segmented structures) and can be a cost-effective method of cladding a shaped glass façade.

Often, cold-bending refers to a practice of fabricating pre-shaped unitised curtain-wall panels and industrial cladding at the production site or the flat glass units are installed at the building site on a curved sub-construction.

A necessary pre-condition is the toughening process of all applied glasses, as the resulting high bending strength of toughened glass allows for applying permanent deflections to the glazing without issue. The radius limitations of the glass units depend on the degree of surface compression. The minimum achievable radii are much larger compared to thermally curved glass and are typically several metres. The typical parameter describing the deflection of cold-bent glass is the displacement „dZ“ (see drawings below).

The shaping of the glass can be achieved by:

pushing by cover bars.

structural sealant between glass and sub-construction.

hardened resin of thin laminated glass (laminate-bending).

The applied permanent tensions of the cold-bent units must be considered for dimensioning the insulating glass sealant depth, together with the bite and area of the used structural silicone if applied on a pre-shaped sub-construction.